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  Java Practices -> Pattern-match lines of a file
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<div class='page-title'>Pattern-match lines of a file</div>

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<br>Here is an example of <tt>grep</tt>-like treatment of the contents
of a text file.
<p>In this example, the format of each line is verified against a regular
expression.
<p>Note that a <tt>String</tt> literal representing a regular expression
needs to escape all backslashes. In effect, all of the backslashes are
simply doubled.

<p><b>Example</b>
<br>
<PRE>

<span class='keyword'>import</span> java.io.*;
<span class='keyword'>import</span> java.util.regex.*;

<span class='keyword'>public</span> <span class='keyword'>class</span> LineMatcher {

  <span class='comment'>/**
  * Verifies that each line of a text file starts with:
  *   &#092;N + tab + integer + tab + text
  * where "text" contains word characters (&#092;w).
  *
  * The corresponding regular expression is:
  *   "^&#092;&#092;N&#092;t(&#092;d)+&#092;t(&#092;w)+"
  *
  * The String passed to Pattern needs to double every backslash:
  *   "^&#092;&#092;&#092;&#092;N&#092;&#092;t(&#092;&#092;d)+&#092;&#092;t(&#092;&#092;w)+"
  *
  * If a line is not of the expected pattern, then an
  * IllegalStateException is thrown.
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>void</span> findBadLines( <span class='keyword'>final</span> File aFile ) {
    <span class='comment'>//...checks on aFile are elided
</span>
    <span class='comment'>//Pattern and Matcher are used here, not String.matches(regexp),
</span>    <span class='comment'>//since String.matches(regexp) would repeatedly compile the same
</span>    <span class='comment'>//regular expression
</span>    Pattern regexp = Pattern.compile(<span class='literal'>"^&#092;&#092;&#092;&#092;N&#092;&#092;t(&#092;&#092;d)+&#092;&#092;t(&#092;&#092;w)+"</span>);
    Matcher matcher = regexp.matcher(<span class='literal'>""</span>);

    LineNumberReader lineReader = <span class='keyword'>null</span>;
    <span class='keyword'>try</span> {
      lineReader = <span class='keyword'>new</span> LineNumberReader( <span class='keyword'>new</span> FileReader(aFile) );
      String line = <span class='keyword'>null</span>;
      <span class='keyword'>while</span> ((line = lineReader.readLine()) != <span class='keyword'>null</span>){
        matcher.reset( line ); <span class='comment'>//reset the input
</span>        <span class='keyword'>if</span> ( !matcher.find() ) {
          String msg = <span class='literal'>"Line "</span> + lineReader.getLineNumber() + <span class='literal'>" is bad: "</span> + line;
          <span class='keyword'>throw</span> <span class='keyword'>new</span> IllegalStateException(msg);
        }
      }
    }
    <span class='keyword'>catch</span> (FileNotFoundException ex) {
      ex.printStackTrace();
    }
    <span class='keyword'>catch</span> (IOException ex){
      ex.printStackTrace();
    }
    <span class='keyword'>finally</span> {
      <span class='keyword'>try</span> {
        <span class='keyword'>if</span> (lineReader!= <span class='keyword'>null</span>) lineReader.close();
      }
      <span class='keyword'>catch</span> (IOException ex) {
        ex.printStackTrace();
      }
    }
  }

  <span class='comment'>/**
  * Test harness.
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>void</span> main (String... arguments) {
    <span class='keyword'>final</span> File file = <span class='keyword'>new</span> File( <span class='literal'>"C:&#092;&#092;Temp&#092;&#092;RegexpTest.txt"</span> );
    LineMatcher.findBadLines( file );
  }
} 
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<div class='topic-section'>See Also :</div>
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  <a href='TopicAction12d8-2.html?Id=42'>Reading and writing text files</a> <br>
 
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